This invention relates in general to transmission lubrication systems and in particular to an improved gerotor pump for use in the lubrication system of a vehicle transmission.
In most vehicles, a transmission is provided in the drive train between the engine and the driven wheels. As is well known, the transmission includes a housing containing an input shaft, an output shaft, and a plurality of meshing gears which are selectively connected between the input shaft and the output shaft. The meshing gears contained within the transmission housing are of varying size so as to provide a plurality of speed reduction gear ratios between the input shaft and the output shaft. By appropriate selection of these meshing gears, a desired speed reduction gear ratio can be obtained between the input shaft and the output shaft. As a result, acceleration and deceleration of the vehicle can be accomplished in a smooth and efficient manner.
Typically, this gear ratio selection is accomplished by moving one or more control members provided within the transmission. Movement of the control member causes certain ones of the meshing gears to be connected between the input shaft and the output shaft so as to provide the desired gear ratio therebetween. In a manual transmission, movement of the control member is accomplished by manual exertion of the vehicle driver, such as through a shift lever. In an automatic transmission, movement of the control member is accomplished by a pnemnatic or hydraulic actuator in response to predetermined operating conditions.
In both manual and automatic transmissions, it is known to provide lubricant within the transmission for minimizing the adverse effects of friction caused by the meshing gears contained therein. In the past, the lower portion of the housing of the transmission functioned as a sump for the lubricant. Portions of the meshing gears of the transmission were partially submerged in the lubricant. Thus, when such gears were rotated during use, the lubricant was carried thereon or splashed into the meshing teeth thereof so as to provide lubrication.
Although this sump type of lubrication system has functioned successfully in the past, it has been found to have several drawbacks. First, a certain amount of clearance space must be provided between the lower portions of the meshing gears and the housing of the transmission. Consequently, a relatively large amount of lubricant is required to fill the sump of the transmission housing to a sufficiently high level to insure that all of the meshing gears are at least partially submerged therein. This relatively large amount of lubricant adds unnecessary weight and expense to the transmission.
Second, transmissions are often operated at an angle which is inclined relative to the horizontal. In some instances, the transmission is installed within the vehicle at an angle which is inclined relative to the horizontal. In other instances, the transmission is installed horizontally within to the vehicle, but the vehicle is operated on an inclined surface, such as when climbing a hill. In either event, because gravity maintains the level of the lubricant horizontal, some of the meshing gears in the transmission are often deeply submerged in the lubricant during operation while others are only partially submerged. The continuous rotation of the deeply submerged gears causes unnecessary churning of the lubricant in the sump, which results in a loss of efficiency and increased lubricant temperature.
The problem of increased lubricant temperature has been addressed in the past by mounting a pump on the exterior of the transmission. The pump is connected to a rotating shaft contained within the transmission so as to be driven thereby. The pump circulates the lubricant out of the sump, through a heat exchanger typically provided at the front of the vehicle, and back into the sump. Unfortunately, this relatively lengthy and complicated fluid path also resulted in loss of efficiency.
To avoid this drawback, a pump has been mounted within the transmission housing to circulate the lubricant. One type of pump which has been mounted within the transmission is a gerotor pump. Gerotor pumps are positive displacement pumps having inner and outer rotors or elements supported within a pump housing. The inner element has one less tooth than the outer element. The inner and outer elements have conjugately-generated tooth profiles to maintain continuous fluid tight contact during operation. As the gerotor revolves, fluid is drawn into the enlarging chamber to a maximum volume equal to that of the missing tooth on the inner element. The fluid is forced out as the teeth mesh, decreasing the chamber volume. Modem vehicle transmissions are becoming increasingly smaller in physical size, and the space available within the transmission case for these pumps is becoming increasingly limited. Thus, while the conventional gerotor pump is efficient and compact relative to other types of pumps, it would be desirable to provide an improved structure for a gerotor pump for a transmission lubrication system which is simple in construction, compact in size and efficient.